arXiv subjects

N. Wang

Publications and source records attributed to N. Wang.

At least 91 records · Page 5Linked to original sources

arXiv2016

Optimization of the collimation system for CSNS/RCS with the robust conjugate direction search algorithm

The Robust Conjugate Direction Search (RCDS) method is used to optimize the collimation system for Rapid Cycling Synchrotron (RCS) of the Chinese Spallation Neutron Source (CSNS). The parameters of secondary collimators are optimized for a better performance of the collimation system. To improve the efficiency of the optimization, the Objective Ring Beam Injection and Tracking (ORBIT) parallel module combined with MATLAB parallel computing is used, which can run multiple ORBIT instances simultaneously. This study presents a way to figure out an optimal parameter combination of the secondary collimators for a machine model in preparation for CSNS/RCS commissioning.

physics.acc-ph↗

arXiv2016

Proper motions of 15 pulsars: a comparison between Bayesian and frequentist algorithms

We present proper motions for 15 pulsars which are observed regularly by the Nanshan 25-m radio telescope. Two methods, the frequentist method (Coles et al.2011) and the Bayesian (Lentati et al. 2014) method, are used and the results are compared. We demonstrate that the two methods can be applied to young pulsar data sets that exhibit large amounts of timing noise with steep spectral exponents and give consistent results. The measured positions also agree with very-long-baseline interferometric positions. Proper motions for four pulsars are obtained for the first time, and improved values are obtained for five pulsars.

astro-ph.IM↗

arXiv2016

The mode switching of PSR B2020+28

This paper reports on polarimetric radiation properties based on the switching modes of normal PSR B2020+28 by analysing the data acquired from the Nanshan 25-m radio telescope at 1556 MHz. With nearly 8 hours quasi-continuous observation, the data presented some striking and updated phenomena. The change of relative intensity between the leading and trailing components is the predominant feature of mode switching. The intensity ratio between the leading and trailing components are measured for the individual profiles averaged over 30 seconds. It is found that there is an excess of high ratios over the normal distribution, which indicates that two modes exist in the pulsar. The distribution of abnormal mode has a narrower width indicating that the abnormal mode is more stable than the normal mode. A total of 76 mode switching events are detected in our data. It spends 89\% in the normal mode and 11\% in the abnormal mode. The intrinsic distributions of mode timescales are constrained with power-law distributions. The significant difference in the index of the duration distribution between normal and abnormal modes possibly indicates that the timescale for the abnormal mode to get stable is shorter than that for the normal mode. The frequent switching between both modes may indicate that the oscillations between different magnetospheric states are rapid.

astro-ph.HE↗

arXiv2016

Investigation of nulling and subpulse drifting properties of PSR J1727$-$2739

Aims. To make a detailed study of the nulling and subpulse drifting in PSR J1727$-$2739 for investigation of its radiation properties. Methods. The observations were carried out on 20 March, 2004 using the Parkes 64-m radio telescope, with a central frequency of 1518 MHz. A total of 5568 single pulses were analysed. Results. This pulsar shows well defined nulls with lengths lasting from 6 to 281 pulses and separated by burst phases ranging from 2 to 133 pulses. We estimate a nulling fraction of around 68\%. No emission in the average pulse profile integrated over all null pulses is detected with significance above $3σ$. Most transitions from nulls to bursts are within a few pulses, whereas the transitions from bursts to nulls exhibit two patterns of decay: decrease gradually or rapidly. In the burst phase, we find that there are two distinct subpulse drift modes with vertical spacing between the drift bands of $9.7 \pm 1.6$ and $5.2 \pm 0.9$ pulse periods, while sometimes there is a third mode with no subpulse drifting. Some mode transitions occur within a single burst, while others are separated by nulls. Different modes have different average pulse profiles. Possible physical mechanisms are discussed.

astro-ph.HE↗

arXiv2016

Modified Fermi Energy of Electrons in a Superhigh Magnetic Field

In this paper, we investigate the electron Landau-level stability and its influence on the electron Fermi energy, $E_{\rm F}(e)$, in the circumstance of magnetars, which are powered by magnetic field energy. In a magnetar, the Landau levels of degenerate and relativistic electrons are strongly quantized. A new quantity $g_{n}$, the electron Landau-level stability coefficient is introduced. According to the requirement that $g_{n}$ decreases with increasing the magnetic field intensity $B$, the magnetic-field index $β$ in the expression of $E_{\rm F}(e)$ must be positive. By introducing the Dirac$-δ$ function, we deduce a general formulae for the Fermi energy of degenerate and relativistic electrons, and obtain a particular solution to $E_{\rm F}(e)$ in a superhigh magnetic field (SMF). This solution has a low magnetic-field index of $β=1/6$, compared with the previous one, and works when $ρ\geq 10^{7}$~g cm$^{-3}$ and $B_{\rm cr}\ll B\leq 10^{17}$~Gauss. By modifying the phase space of relativistic electrons, a SMF can enhance the electron number density $n_e$, and decrease the maximum of electron Landau level number, which results in a redistribution of electrons. According to Pauli exclusion principle, the degenerate electrons will fill quantum states from the lowest Landau level to the highest Landau level. As $B$ increases, more and more electrons will occupy higher Landau levels, though $g_{n}$ decreases with the Landau level number $n$. The enhanced $n_{e}$ in a SMF means an increase in the electron Fermi energy and an increase in the electron degeneracy pressure. The results are expected to facilitate the study of the weak-interaction processes inside neutron stars and the magnetic-thermal evolution mechanism for megnetars.

astro-ph.HE↗

arXiv2015

The Effects of Superhigh Magnetic Fields on Equations of States of Neutron Stars

By introducing Dirac ?-function in superhigh magnetic field, we deduce a general formula for pressure of degenerate and relativistic electrons, Pe, which is suitable for superhigh magnetic fields, discuss the quantization of Landau levels of electrons, and consider the quantum electrodynamic(QED) effects on the equations of states (EOSs) for different matter systems. The main conclusions are as follows: the stronger the magnetic field strength, the higher the electron pressure becomes; compared with a common radio pulsar, a magnetar could be a more compact oblate spheroid-like deformed neutron star due to the anisotropic total pressure; and an increase in the maximum mass of a magnetar is expected because of the positive contribution of the magnetic field energy to the EOS of the star. Since this is an original work in which some uncertainties could exist, to further modify and perfect our theory model should be considered in our future studies.

hep-ph↗

arXiv2015

Constraining the Braking Indices of Magnetars

Due to the lack of long term pulsed emission in quiescence and the strong timing noise, it is impossible to directly measure the braking index $n$ of a magnetar. Based on the estimated ages of their potentially associated supernova remnants (SNRs), we estimate the values of the mean braking indices of eight magnetars with SNRs, and find that they cluster in a range of $1\sim$42. Five magnetars have smaller mean braking indices of $1 3$ for other three magnetars are attributed to the decay of external braking torque, which might be caused by magnetic field decay. We estimate the possible wind luminosities for the magnetars with $1 3$ within the updated magneto-thermal evolution models. Although the constrained range of the magnetars' braking indices is tentative, due to the uncertainties in the SNR ages, which come from distance uncertainties and the unknown conditions of the expanding shells, our method provides an effective way to constrain the magnetars' braking indices if the measurements of the SNRs' ages are reliable, which can be improved by future observations.

astro-ph.HE↗

arXiv2015

Mass dependence of symmetry energy coefficients in Skyrme force

Based on the semi-classical extended Thomas-Fermi approach, we study the mass dependence of the symmetry energy coefficients of finite nuclei for 36 different Skyrme forces. The reference densities of both light and heavy nuclei are obtained. Eight models based on nuclear liquid drop concept and the Skyrme force SkM* suggest the symmetry energy coefficient $a_{\rm sym}=22.90 \pm 0.15 $ MeV at $A=260$, and the corresponding reference density is $ρ_A\simeq 0.1$ fm$^{-3}$ at this mass region. The standard Skyrme energy density functionals give negative values for the coefficient of the $I^4$ term in the binding energy formula, whereas the latest Weizsäcker-Skyrme formula and the experimental data suggest positive values for the coefficient.

nucl-th↗

arXiv2015

Pulsar wind model for the spin-down behavior of intermittent pulsars

Intermittent pulsars are part-time radio pulsars. They have higher slow down rate in the on state (radio-loud) than in the off state (radio-quiet). This gives the evidence that particle wind may play an important role in pulsar spindown. The effect of particle acceleration is included in modeling the rotational energy loss rate of the neutron star. Applying the pulsar wind model to the three intermittent pulsars (PSR B1931+24, PSR J1841-0500, and PSR J1832+0029), their magnetic field and inclination angle are calculated simultaneously. The theoretical braking indices of intermittent pulsars are also given. In the pulsar wind model, the density of the particle wind can always be the Goldreich-Julian density. This may ensure that different on states of intermittent pulsars are stable. The duty cycle of particle wind can be determined from timing observations. It is consistent with the duty cycle of the on state. Inclination angle and braking index observations of intermittent pulsars may help to test different models of particle acceleration. At present, the inverse Compton scattering induced space charge limited flow with field saturation model can be ruled out.

astro-ph.HE↗

arXiv2014

Searching for gravitational wave memory bursts with the Parkes Pulsar Timing Array

Anisotropic bursts of gravitational radiation produced by events such as super-massive black hole mergers leave permanent imprints on space. Such gravitational wave "memory" (GWM) signals are, in principle, detectable through pulsar timing as sudden changes in the apparent pulse frequency of a pulsar. If an array of pulsars is monitored as a GWM signal passes over the Earth, the pulsars would simultaneously appear to change pulse frequency by an amount that varies with their sky position in a quadrupolar fashion. Here we describe a search algorithm for such events and apply the algorithm to approximately six years of data from the Parkes Pulsar Timing Array. We find no GWM events and set an upper bound on the rate for events which could have been detected. We show, using simple models of black hole coalescence rates, that this non-detection is not unexpected.

astro-ph.GA↗

arXiv2014

Linear Fringe Field Effects of Quadrupoles

Fringe field becomes important when one requires more accurate modeling of a ring lattice to study the long-term beam dynamics in storage rings and deal with large aperture magnets in high-intensity proton synchrotrons or accumulator rings. In this paper, a simple expression to calculate the tune shifts due to quadrupole fringe fields is derived by using Lie algebra technique. With higher-order terms included, this method is more accurate compared with the linear fringe field model used in SAD code. The method is also applied to a BEPCII lattice. Also based on the Lie algebra technique and an inverse series technique, an equivalent hard-edge model for quadrupoles is proposed, in which the model parameters are derived analytically. The model has the advantages of the direct calculation of the equivalent length and strength of a quadrupole and the easy adaptation when the strength changes. The validity of the model and its simplified version has been checked with a numerical method, and they show very good agreements.

physics.acc-ph↗

arXiv2013

Numerical simulation of the electron capture process in a magnetar interior

In a superhigh magnetic field, direct Urca reactions can proceed for an arbitrary proton concentration. Since only the electrons with high energy $E$ ($E > Q$, $Q$ is the threshold energy of inverse $β-$decay) at large Landau levels can be captured, we introduce the Landau level effect coefficient $q$ and the effective electron capture rate $Γ_{\rm eff}$. By using $Γ_{\rm eff}$, the values of $L_{\rm X}$ and $L_{\rm ν}$ are calculated, where and $L_{\rm ν}$, $L_{\rm X}$ are the average neutrino luminosity of Anomalous X-ray Pulsars (AXPs) and the average X-ray luminosity of AXPs, respectively. The complete process of electron capture inside a magnetar is simulated numerically.

astro-ph.HE↗

arXiv2013

Evolution of superhigh magnetic fields of magnetars

In this paper, we consider the effect of Landau levels on the decay of superhigh magnetic fields of magnetars. Applying ${}^3P_2$ anisotropic neutron superfluid theory yield a second-order differential equation for a superhigh magnetic field $B$ and its evolutionary timescale $t$. The superhigh magnetic fields may evolve on timescales $\sim (10^{6}-10^{7})$ yrs for common magnetars. According to our model, the activity of a magnetar may originate from instability caused by the high electron Fermi energy.

astro-ph.HE↗

arXiv2013

Landau level-superfluid modified factor and effective X/$γ$-ray coefficient of a magnetar

As soon as the energy of electrons near the Fermi surface are higher than $Q$, the threshold energy of inverse $β-$ decay, the electron capture process will dominate. The resulting high-energy neutrons will destroy anisotropic ${}^3P_2$ neutron superfluid Cooper pairs. By colliding with the neutrons produced in the process $n+ (n\uparrow n\downarrow)\longrightarrow n+ n+ n$, the kinetic energy of the outgoing neutrons will be transformed into thermal energy. The transformed thermal energy would transported from the star interior to the star surface by conduction, then would be transformed into radiation energy as soft X-rays and gamma-rays. After a highly efficient modulation within the pulsar magnetosphere, the surface thermal emission (mainly soft X/$γ$-ray emission) has been shaped into a spectrum with the observed characteristics of magnetars. By introducing two important parameters: Landau level-superfluid modified factor and effective X/$γ$-ray coefficient, we numerically simulate the process of magnetar cooling and magnetic field decay, and then compute magnetars' soft X/$γ$-ray luminosities $L_{X}$. Further, we obtain aschematic diagrams of $L_{X}$ as a function of magnetic field strength $B$. The observations are compared with the calculations.

astro-ph.HE↗

arXiv2013

Magnetic field decay of magnetars in supernova remnants

In this paper, we modify our previous research carefully, and derive a new expression of electron energy density in superhigh magnetic fields. Based on our improved model, we re-compute the electron capture rates and the magnetic fields' evolutionary timescales $t$ of magnetars. According to the calculated results, the superhigh magnetic fields may evolve on timescales $\sim (10^{6}-10^{7})$ yrs for common magnetars, and the maximum timescale of the field decay, $t\approx 2.9507 \times 10^{6}$ yrs, corresponding to an initial internal magnetic field $B_{\rm 0}= 3.0 \times 10^ {15}$ G and an initial inner temperature $T_{\rm 0}= 2.6 \times 10^ {8}$ K. Motivated by the results of the neutron star-supernova remnant(SNR) association of Zhang $\&$ Xie(2011), we calculate the maximum $B_{\rm 0}$ of magnetar progenitors, $B_{\rm max}\sim (2.0\times 10^{14}-2.93 \times 10^{15})$ G when $T_{\rm 0}= 2.6 \times 10^ {8}$ K. When $T_{\rm 0}\sim 2.75 \times 10^ {8}-~1.75 \times 10^ {8}$ K, the maximum $B_{\rm 0}$ will also be in the range of $\sim 10^{14}-10^{15}$ G, not exceeding the upper limit of magnetic field of a magnetar under our magnetar model. We also investigate the relationship between the spin-down ages of magnetars and the ages of their SNRs, and explain why all AXPs associated with SNRs look older than their real ages, whereas all SGRs associated with SNRs appear younger than they are.

astro-ph.HE↗

arXiv2013

The surface and inner temperatures of magnetars

Assuming that the timescale of the magnetic field decay is approximately equal to that of the stellar cooling via neutrino emission, we obtain a one-to-one relationship between the effective surface thermal temperature and the inner temperature. The ratio of the effective neutrino luminosity to the effective X-ray luminosity decreases with decaying magnetic field.

astro-ph.HE↗

arXiv2013

A new method to analyse pulsar nulling phenomenon

Pulsar nulling is a phenomenon of sudden cessation of pulse emission for a number of periods. The nulling fraction was often used to characterize the phenomenon. We propose a new method to analyse pulsar nulling phenomenon, by involving two key parameters, the nulling degree, $χ$, which is defined as the angle in a rectangular coordinates for the numbers of nulling periods and bursting periods, and the nulling scale, $ N $, which is defined as the effective length of the consecutive nulling periods and bursting periods. The nulling degree $χ$ can be calculated by $\tan χ= N_{\rm nulling} / N_{\rm bursting} $ and the mean is related to the nulling fraction, while the nulling scale, $ N $, is also a newly defined fundamental parameter which indicates how often the nulling occurs. We determined the distributions of $χ$ and $ N $ for 10 pulsars by using the data in literature. We found that the nulling degree $χ$ indicates the relative length of nulling to that of bursting, and the nulling scale $ N $ is found to be related to the derivative of rotation frequency and hence the loss rate of rotational energy of pulsars. Their deviations reflect the randomness of the nulling process.

astro-ph.HE↗

arXiv2013

The effects of intense magnetic fields on Landau levels in a neutron star

In this paper, an approximate method of calculating the Fermi energy of electrons ($E_{F}(e)$) in a high-intensity magnetic field, based on the analysis of the distribution of a neutron star magnetic field, has been proposed. In the interior of a Neutron star, different forms of intense magnetic field could exist simultaneously and a high electron Fermi energy could be generated by the release of magnetic field energy. The calculation results show that: $E_{F}(e)$ is related to density $ρ$, the mean electron number per baryon $Y_{e}$ and magnetic field strength $B$.

astro-ph.HE↗